Method and a device for preparing a medical liquid

ABSTRACT

A method for preparing a medical liquid from a liquid, such as water, and two concentrated solutions comprises the following steps:  
     circulating the liquid in a conduit ( 15 ), at a flowrate Q 0;    
     injecting into the conduit ( 15 ), at a flowrate Q 1 , a first concentrated solution containing a first ionic substance A and a second ionic substance B, the ionic substances A and B having, respectively, in the first concentrated solution, a concentration [Asol] and a first concentration [B 1 sol];  
     injecting into the conduit ( 15 ), at a flowrate Q 2 , a second concentrated solution containing the first ionic substance A and the second ionic substance B, the first ionic substance A having, in the second concentrated solution, the same concentration [Asol] as in the first concentrated solution, and the second ionic substance B having, in the second concentrated solution, a second concentration [B 2 sol] different than the first concentration [B 1 sol] in the first concentrated solution;  
     regulating the injection flowrate Q 1  and the injection flowrate Q 2  of the first and second concentrated solutions in such a way that at any given time the diluted solution resulting from the mixing of the liquid and the concentrated solutions has a desired concentration [Ades] of first substance A and a desired concentration [Bdes] of second substance B.

[0001] The present invention relates to a method and a device forpreparing a medical liquid.

[0002] The invention has an application in particular in the treatmentof renal insufficiency, where it can be used for preparing a dialysisliquid. In this context, the invention is particularly suitable fortreating patients whose internal medium presents an excess of potassium.

[0003] The kidneys perform many functions, including elimination ofwater, excretion of catabolites (or waste products of metabolism, suchas urea and creatinine), regulation of the concentration of electrolytesin the blood (sodium, potassium, magnesium, calcium, bicarbonates,phosphates, chlorides), and regulation of the acid-base balance of theinternal medium, which balance is obtained in particular through theelimination of weak acids (phosphates, monosodium acids) and through theproduction of ammonium salts.

[0004] In persons who have lost the use of their kidneys, since theseexcretory and regulatory mechanisms no longer function, the internalmedium becomes charged with water and waste products of metabolism andpresents an excess of electrolytes (sodium in particular), and, ingeneral, acidosis, with the pH of the blood plasma shifting towards 7.

[0005] To remedy kidney dysfunction, the conventional practice is totreat the blood by extracorporeal circulation in a semipermeablemembrane exchanger (hemodialyzer), with circulation, on either side ofthe membrane, of the patient's blood and of a dialysis liquid comprisingthe main electrolytes of the blood (chlorides, bicarbonates, sodium,calcium, potassium, magnesium) in concentrations close to those of theblood of a healthy subject. As a result of the physical phenomenoncalled dialysis, the molecules migrate from the liquid in which theirconcentration is highest to the liquid in which their concentration islowest.

[0006] A significant electrolytic change in uremic patients is theincrease in the potassium concentration of the plasma. Now, hyperkalemia(too high a concentration of potassium) is associated with incidentslinked to hyperpolarization of the membrane of the neuromuscular cells,which can result in hypokinetic arrhythmia and complete atrioventricularblock. One of the objectives of dialysis treatment is therefore toeliminate the excess potassium accumulated by the patients between twotreatment sessions. In accordance with the physical principle citedabove, the quantity of potassium eliminated during treatment dependsdirectly on the difference between the concentration of the potassium inthe plasma and the concentration of the potassium in the dialysisliquid, which is generally fixed at a constant level, less(approximately 2 mEq/l) than the physiological level (approximately 3.5mEq/l).

[0007] At the start of conventional dialysis treatment, a patient withhyperkalemia (whose plasma potassium concentration can be as high as 10mEq/l) is exposed to the undesirable effects resulting from theconsiderable difference between the potassium concentration of hisplasma and that of the dialysis liquid: this increased gradient in factcauses a substantial diffusive flow of potassium across the membrane ofthe hemodialyzer, which in turn causes a substantial flow of potassiumacross the membrane of the cells, which affects the electric potentialof the membrane at rest and, consequently, the cellular excitability. Asthis mechanism also influences the cardiac pacemaker cells, the patentruns the risk of cardiac arrhythmia during the dialysis treatment. Thisphenomenon is naturally heightened in cases of cardiac weakness and canlead to a reduction in the ejection volume affecting the cardiovascularcirculation.

[0008] A particular object of the invention is therefore to modify theconditions of conventional dialysis treatment, without however affectingits effectiveness, in such a way that patients with hyperkalemia are nolonger exposed to the risks mentioned above.

[0009] A general object of the invention is to conceive a device and amethod for preparing a treatment liquid which can be used forextracorporeal treatment of blood, and by means of which theconcentration of two ionic substances can be adjusted separately, inparticular sodium and potassium (or calcium, or magnesium).

[0010] According to the invention, this object is achieved by means of amethod for preparing a medical liquid from a liquid, such as water, andtwo concentrated solutions, comprising the following steps:

[0011] circulating the liquid in a conduit, at a flowrate Q0;

[0012] injecting into the conduit, at a flowrate Q1, a firstconcentrated solution containing a first ionic substance A and a secondionic substance B, the ionic substances A and B having, respectively, inthe first concentrated solution, a concentration [Asol] and a firstconcentration [B1sol];

[0013] injecting into the conduit, at a flowrate Q2, a secondconcentrated solution containing the first ionic substance A and thesecond ionic substance B, the first ionic substance A having, in thesecond concentrated solution, the same concentration [Asol] as in thefirst concentrated solution, and the second ionic substance B having, inthe second concentrated solution, a second concentration [B2sol]different than the first concentration [B1sol] in the first concentratedsolution;

[0014] regulating the injection flowrate Q1 and the injection flowrateQ2 of the first and second concentrated solutions in such a way that atany given time the diluted solution resulting from the mixing of theliquid and the concentrated solutions has a desired concentration [Ades]of first substance A and a desired concentration [Bdes] of secondsubstance B.

[0015] According to one characteristic of the invention, the injectionflowrate Q1 and the injection flowrate Q2 of the concentrated solutionsA and B are varied over the course of time in such a way that theconcentration of the second substance B in the diluted solution variesover the course of time in accordance with a predetermined profile.

[0016] According to another characteristic of the invention, theflowrate Q0 of the liquid in the conduit is constant, and the sum of theinjection flowrates Q1+Q2 of the concentrated solutions A and B ismaintained constant in such a way that the concentration of the firstsubstance A in the diluted solution remains substantially constant.

[0017] According to yet another characteristic of the invention, theinjection flowrate Q1 and the injection flowrate Q2 of the concentratedsolutions A and B are varied over the course of time in such a way thatthe concentration of the first substance A in the diluted solutionvaries over the course of time in accordance with a predeterminedprofile.

[0018] The invention also relates to a device for preparing a treatmentliquid from a liquid, such as water, and two concentrated solutions,comprising:

[0019] a conduit with a first end intended to be connected to a sourceof liquid, such as water, and a second end for delivering a treatmentliquid;

[0020] first injection means for injecting into the conduit, at aflowrate Q1, a first concentrated solution containing a first ionicsubstance A and a second ionic substance B, the ionic substances A and Bhaving, respectively, in the first concentrated solution, aconcentration [Asol] and a first concentration [B1sol];

[0021] second injection means for injecting into the conduit, at aflowrate Q2, a second concentrated solution containing the first ionicsubstance A and the second ionic substance B, the first ionic substanceA having, in the second concentrated solution, the same concentration[Asol] as in the first concentrated solution, and the second ionicsubstance B having, in the second concentrated solution, a secondconcentration [B2sol] different than the first concentration [B1sol] inthe first concentrated solution;

[0022] regulating means for regulating the first and second injectionmeans and for adjusting the injection flowrate Q1 and the injectionflowrate Q2 of the first and second concentrated solutions in such a waythat at any, given time the diluted solution resulting from the mixingof the liquid and the concentrated solutions has a desired concentration[Ades] of first substance A and a desired concentration [Bdes] of secondsubstance B.

[0023] In one embodiment of the invention, the device for preparingtreatment liquid is incorporated in a hemodialysis system, the substanceA is sodium and the substance B is potassium, calcium, or magnesium. Asthe sodium concentration in a dialysis liquid is much higher than thepotassium (calcium or magnesium) concentration, the potassiumconcentration can be very precisely regulated by measuring theconductivity of the mixture forming in the conduit immediatelydownstream of the site of injection of the first concentrated solutioninto the conduit, and of the mixture forming in the conduit immediatelydownstream of the site of injection of the second concentrated solutioninto the conduit (there is an excellent correlation between theconductivity of a solution and its sodium concentration).

[0024] Moreover, there is no danger of influencing the plasma potassiumor calcium concentration of a patient using a very dilute dialysissolution prepared and administered by a system equipped with reliablemeans for measuring concentrations, which would not be the case if thisobjective were achieved by injection of more concentrated solution.

[0025] The invention also relates to a kit of solutions forextracorporeal treatment of blood, comprising two concentrated solutionsand a bag with two compartments intended to contain each of thesolutions from the kit. Each of the solutions contains at least twoionic substances A and B, the ionic substance A having the sameconcentration in the two solutions and the ionic substance B havingdifferent concentrations in two solutions.

[0026] According to one characteristic of the invention, the twosolutions are identical except for one ionic substance whoseconcentration differs from one solution to the other.

[0027] Other characteristics and advantages of the invention will becomemore apparent on reading the following description. Reference will bemade to the attached drawings, in which:

[0028]FIG. 1 is a diagram showing a device for treatment of blood;

[0029]FIG. 2 shows a bag with two compartments for containing the twoconcentrated solutions from a treatment kit; and

[0030]FIG. 3 is a graph showing several profiles of variation in thepotassium concentration of a dialysis liquid.

[0031] The hemodialysis system shown in FIG. 1 comprises a hemodialyzer1 with two compartments 2, 3 separated by a semipermeable membrane 4. Afirst compartment 2 has an inlet connected to a blood withdrawal conduit5 on which a circulation pump 6 is arranged, and an outlet connected toa blood return conduit 7 on which a bubble trap 8 is interposed.

[0032] An infusion device comprising a pump 10 and a balance 11 isprovided for injecting into the bubble trap 8 the contents of a bag 9 ofinfusion liquid containing sodium bicarbonate. The bag 9 is suspendedfrom the balance 11 and it is connected to the bubble trap 8 via aconduit 12 on which the infusion pump 10 is arranged. The balance 11serves to control the pump 10 so that the flowrate of the infusionliquid is equal to a reference flowrate.

[0033] The second compartment 3 of the hemodialyzer 1 has an inletconnected to a conduit 12 for supply of fresh dialysis liquid, and anoutlet connected to a conduit 13 for removal of spent liquid (dialysisliquid and ultrafiltrate).

[0034] The supply conduit 12 connects the hemodialyzer 1 to a device 14for preparing dialysis liquid, comprising a main conduit 15 whoseupstream end is intended to be connected to a source of running water.First and second subsidiary channels 16, 17 are connected to this mainconduit 15.

[0035] According to the invention, the free end of the first subsidiaryconduit 16 is intended to be immersed in a container 18 for a firstconcentrated saline solution containing sodium chloride, calciumchloride, magnesium chloride and potassium chloride. This first conduit16 is equipped with a pump 19 for metering the first concentratedsolution into the dialysis liquid, which is controlled as a function ofthe comparison between 1) a first reference value of conductivity forthe solution forming at the junction of the main conduit 15 and thefirst subsidiary conduit 16, and 2) the value of the conductivity ofthis solution measured by means of a first conductivity probe 20arranged on the main conduit 15 immediately downstream of the junctionbetween the main conduit 15 and the first subsidiary conduit 16.

[0036] According to the invention, the free end of the second subsidiaryconduit 17 is intended to be immersed in a container 21 for a secondconcentrated saline solution containing sodium chloride, calciumchloride, magnesium chloride and potassium chloride. This secondsolution contains the same ionic substances as the first solution and inthe same concentrations, except for potassium whose concentration isdifferent. The second conduit 17 is equipped with a pump 22 for meteringthe second concentrated solution into the dialysis liquid, which iscontrolled as a function of the comparison between 1) a second referencevalue of conductivity for the solution forming at the junction of themain conduit 15 and the second subsidiary conduit 17, and 2) the valueof the conductivity of this solution measured by means of a secondconductivity probe 23 arranged on the main conduit 15 immediatelydownstream of the junction between the main conduit 15 and thesecond'subsidiary conduit 17.

[0037] The conduit 12 for supply of dialysis liquid forms thecontinuation of the main conduit 15 of the device 14 for preparation ofdialysis liquid. Arranged on this supply conduit 12, in the direction ofcirculation of the liquid, are a first flowmeter 24 and a firstcirculation pump 25.

[0038] The downstream end of the conduit 13 for removal of spent liquidis intended to be connected to the drain. Arranged on this conduit, inthe direction of circulation of the liquid, are a probe 26 for measuringthe potassium concentration, a second circulation pump 27, and a secondflowmeter 28. An extraction pump 29 is connected to the removal conduit13 upstream of the second circulation pump 27.

[0039] The hemodialysis system represented in FIG. 1 also comprises acalculation and control unit 30. This unit is connected to a screen 31and to a keyboard 32 via which the user inputs various reference values:flowrate reference values (blood flowrate Qb, dialysis liquid flowrateQd, infusion solution flowrate Qinf), reference values for concentrationof ionic substances in the dialysis liquid, reference value for theduration of treatment T, reference: value for loss of weight WL.Moreover, the calculation and control unit 30 receives informationemitted by the measurement elements of the system, such as theflowmeters 24, 28, the conductivity probes 20, 23, and the probe 26 formeasuring potassium concentration. As a function of the instructionsreceived and of programmed operating modes and algorithms, it controlsthe drive elements of the system, such as the pumps 6, 10, 19, 22, 25,27, 29.

[0040] According to the invention, the concentration of sodium and theconcentration of potassium in the dialysis liquid can be adjustedindependently of one another: for a constant flowrate Q0 of water, theconcentration of sodium depends on the sum of the flowrate Q1 of thefirst concentrated solution injected via the pump 19 into the mainconduit 15 and the flowrate Q2 of the second concentrated solutioninjected via the pump 22 into the main conduit 15, while theconcentration of potassium depends on the ratio of the flowrates Q1, Q2of the first and second concentrated solutions. The concentration ofsodium and of potassium in the dialysis liquid is chosen as a functionof each individual patient. It can be set at a fixed value. According tothe invention, for patients with hyperkalemia, the potassiumconcentration of the dialysis liquid is modified continuously during thetreatment session according to a predetermined profile of variation.

EXAMPLE

[0041] The hemodialysis system described above is equipped with a bag 50made of flexible transparent plastic, as is represented in FIG. 2, andcomprising two compartments 51 and 52 corresponding respectively to thecontainers 18 and 21 in FIG. 1. The bag 50 is provided in its upper partwith eyelets 53 by which it can be suspended vertically from a suitablesupport. Each compartment 51, 52 is equipped at its base with an accesstube 54, 55 provided at its end with a connection element 56, 57intended to cooperate with a complementary connection element fixed tothe end of the subsidiary conduits 16, 17 of the device 14 forpreparation of dialysis liquid. A clip 58, 59 is arranged on each of thetubes 54, 55.

[0042] The compartment 51 (container 18) contains the followingsubstances, in the following concentrations:

[0043] NaCl: 284.31 g/l

[0044] KCl: 19.57 g/l

[0045] CaCl2: 10.29 g/l

[0046] MgC12: 2.63 g/l

[0047] Anhydrous glucose: 35 g/l.

[0048] The compartment 52 (container 21) contains the followingsubstances, in the following concentrations:

[0049] NaCl: 284.31 g/l

[0050] KCl: 0 g/l

[0051] CaCl2: 10.29 g/l

[0052] MgCl2: 2.63 g/l

[0053] Anhydrous glucose: 35 g/l.

[0054] By means of these two solutions it is possible, according to theinvention, to prepare a dialysis liquid having a sodium concentration ofbetween approximately 130 mEq/l and approximately 155 mEq/l, and apotassium concentration varying, during a treatment session, frombetween an initial value of approximately 2.5 mEq/l and approximately5.5 mEq/l and a final value of between approximately 1 mEq/l andapproximately 2 mEq/l.

[0055]FIG. 3 shows four profiles of variation of the potassiumconcentration of a dialysis liquid, which profiles can be obtained usingthe device 14 for preparation of dialysis liquid connected to the bag 50with two compartments containing the concentrated solutions which havejust been described. In this figure, the broken line shows the constantpotassium concentration of a conventional dialysis liquid, that is tosay 2 mEq/l.

[0056] The hemodialysis apparatus which has just been describedfunctions in the following manner.

[0057] An operator inputs to the control unit 30, via the keyboard 32,conventional reference values corresponding to the various parameters oftreatment (prescription), namely the blood flowrate Qb, the dialysisliquid flowrate Qd, the infusion flowrate Qinf of the bicarbonatesolution, the total weight loss WL (quantity of plasma water to bewithdrawn from patient by ultrafiltration), the total duration T of thesession, and the sodium concentration of the dialysis liquid.

[0058] According to the invention, the operator also inputs, to thecontrol unit, an information item or a series of information itemsconcerning the potassium concentration of the dialysis liquid, which canbe either a fixed reference value or one of the variation profilesstored beforehand in the control unit, corresponding for example to oneof the graphs in FIG. 3. The operator can also create and store aprofile appropriate to an individual client.

[0059] According to an alternative embodiment of the invention, thepotassium concentration of the dialysis liquid is adjusted via thecontrol unit 30 in the following manner: a dialysis-liquid having apotassium concentration corresponding to a predetermined reference valueis initially circulated in the hemodialyzer 1, and this reference valueis compared with the value of the potassium concentration in the spentliquid, measured by the probe 26. The control unit 30 subsequentlycontrols the pumps 19, 22 of the device 14 for preparation of treatmentliquid in such a way that the difference between the reference value andthe measured value remains substantially equal to a given value,corresponding to a difference, acceptable for the patient, between thepotassium concentration of the plasma and that of the dialysis liquid.

[0060] After a kit of concentrated solutions, such as the bag describedabove, has been connected to the conduits 16, 17 of the device 14 forpreparation of dialysis liquid, the dialysis liquid circuit is filledwith dialysis liquid. To do this, the main conduit 15 is connected to asource of running water and the pumps 19, 22, 25, 27 are started up. Thepumps 19 and 22 are regulated via the control unit 30 in such a way thatthe potassium concentration and the sodium concentration of the dialysisliquid are equal to the corresponding reference values. The pumps 25, 27for circulating dialysis liquid are regulated via the control unit 30 in'such a way that the flowrate of the pump 25 situated upstream of thehemodialyzer 1 is equal to the reference flowrate Qd (500 ml/min, forexample) and so that the flowrate of the pump 27 situated downstream ofthe hemodialyzer 1 is such that the flowrates measured by the flowmeters24, 28 are equal.

[0061] At the same time as the dialysis liquid circuit is filling withthe dialysis liquid according to the prescription, the circuit forextracorporeal blood circulation is rinsed and filled with sterilephysiological liquid.

[0062] When priming of the dialysis liquid circuit and of the bloodcircuit is completed, the blood circuit is connected to the patient andthe treatment proper can commence: the pumps 19, 22 of the device 14 forpreparation of dialysis liquid, and the pumps 25, 27 for circulating thedialysis liquid, continue functioning, while the blood pump 6, theextraction pump 29 and the infusion pump 10 are started up. The bloodpump 6 is set at the reference flowrate Qb (for example 200 ml/mn), theinfusion pump 10 is set at the reference flowrate Qinf, and theextraction pump 29 is set at a flowrate QUF calculated by the controlunit 30 on the basis of the reference values for total weight loss WL,infusion flowrate Qinf and total duration of treatment T.

[0063] The invention which has just been described is open to variants.

[0064] In the same way as the potassium concentation, the calcium ormagnesium concentration can be adjusted to the needs of each individualpatient.

[0065] With the preparation device according to the invention, it ispossible to simultaneously adjust the potassium concentration of adialysis liquid according to a first defined variation profile and thesodium concentration of the same dialysis liquid according to a seconddefined variation profile.

[0066] A probe for measuring the potassium concentration can be mountedon the supply conduit 12 in order to provide a measured value of thepotassium concentration which will be used, for example, to calculatethe difference between this value and the value measured downstream ofthe hemodialyzer 1 by the probe 26.

1-17. (Canceled).
 18. Method for preparing a medical liquid from aliquid and two concentrated solutions, comprising the following steps:circulating the liquid in a preparation conduit; injecting into thepreparation conduit, at a first injection flowrate Q1, a firstconcentrated solution containing a first ionic substance A and a secondionic substance B, the second ionic substance B being potassium;injecting into the preparation conduit, at a second injection flowrateQ2, a second concentrated solution containing the first ionic substanceA, the concentration of the second ionic substance B in the secondconcentrated solution being equal to zero; and regulating the firstinjection flowrate Q1 and the second injection flowrate Q2 in such a waythat at any given time the diluted solution resulting from the mixing ofthe liquid and the concentrated solutions has a desired concentration ofthe first ionic substance A and a desired concentration of the secondionic substance B.
 19. Method according to claim 18, further comprisingvarying over the course of time the first injection flowrate Q1 in sucha way that the concentration of the second ionic substance B in thediluted solution varies over the course of time in accordance with asecond concentration profile.
 20. Method according to claim 19, whereinthe second concentration profile is a descending profile whose initialvalue is between approximately 2.5 mEq/l and approximately 5.5 mEq/l andwhose final value is between approximately 1 mEq/l and approximately 2mEq/l.
 21. Method according to claim 18, further comprising varying overthe course of time the first injection flowrate Q1 and the secondinjection flowrate Q2 of the concentrated solutions in such a way thatthe concentration of the first ionic substance A in the diluted solutionvaries over the course of time in accordance with a first concentrationprofile.
 22. Method according to claim 21, wherein the firstconcentration profile is a constant profile according to which theconcentration of the first ionic substance A in the diluted solutionremains substantially constant.
 23. Method according to claim 18,further comprising varying over the course of time the first injectionflowrate Q1 and the second injection flowrate Q2 of the concentratedsolutions in such a way that the concentration of the first ionicsubstance A in the diluted solution varies over the course of time inaccordance with a first concentration profile and the concentration ofthe second ionic substance B in the diluted solution varies over thecourse of time in accordance with a second concentration profile. 24.Method according to claim 18, wherein the liquid is circulated in thepreparation conduit at a constant flowrate Q0.
 25. Method according toclaim 24, wherein the sum of the injection flowrates Q1+Q2 of theconcentrated solutions is maintained constant in such a way that theconcentration of the first ionic substance A in the diluted solutionremains substantially constant.
 26. Method according to claim 18,wherein the concentration of the first ionic substance A in the firstconcentrated solution is equal to the concentration of the first ionicsubstance A in the second concentrated solution.
 27. Method according toclaim 18, wherein the concentration of the first ionic substance A inthe first concentrated solution is less than twice the concentration ofthe first ionic substance A in the second concentrated solution. 28.Method according to claim 18, wherein the first ionic substance A issodium.
 29. Method for preparing a medical liquid from a liquid and twoconcentrated solutions, comprising the following steps: circulating theliquid in a preparation conduit; injecting into the preparation conduit,at a first injection flowrate Q1, a first concentrated solutioncontaining at least a first ionic substance A and a second ionicsubstance B; injecting into the preparation conduit, at a secondinjection flowrate Q2, a second concentrated solution, the twoconcentrated solutions being identical to each other except that theconcentration of the second ionic substance B differs from one solutionto the other; and regulating the first injection flowrate Q1 and thesecond injection flowrate Q2 in such a way that at any given time thediluted solution resulting from the mixing of the liquid and theconcentrated solutions has a desired concentration of the first ionicsubstance A and a desired concentration of the second ionic substance B.30. Method according to claim 29, further comprising varying over thecourse of time the first injection flowrate Q1 and the second injectionflowrate Q2 of the concentrated solutions in such a way that theconcentration of the second ionic substance B in the diluted solutionvaries over the course of time in accordance with a predetermined firstconcentration profile.
 31. Method according to claim 30, wherein thesecond ionic substance B is potassium, and the predetermined firstconcentration profile is a descending profile whose initial value isbetween approximately 2.5 mEq/l and approximately 5.5 mEq/l and whosefinal value is between approximately 1 mEq/l and approximately 2 mEq/l.32. Method according to claim 29, further comprising varying over thecourse of time the first injection flowrate Q1 and the second injectionflowrate Q2 of the concentrated solutions in such a way that theconcentration of the first ionic substance A in the diluted solutionvaries over the course of time in accordance with a first concentrationprofile.
 33. Method according to claim 32, wherein the firstconcentration profile is a constant profile according to which theconcentration of the first ionic substance A in the diluted solutionremains substantially constant.
 34. Method according to claim 29,wherein the liquid is circulated in the preparation conduit at aconstant flowrate Q0.
 35. Method according to claim 34, wherein the sumof the injection flowrates Q1+Q2 of the concentrated solutions ismaintained constant in such a way that the concentration of the firstionic substance A in the diluted solution remains substantiallyconstant.
 36. Method according to claim 29, wherein the first ionicsubstance A is sodium, and the second ionic substance B is one ofpotassium, calcium, and magnesium.
 37. System for extracorporealtreatment of blood comprising: a preparation conduit with a first endintended to be connected to a source of liquid and a second end fordelivering a treatment liquid; a circulation device for circulating theliquid in the preparation conduit; a first injection device forinjecting into the preparation conduit, at a first injection flowrateQ1, a first concentrated solution containing at least a first ionicsubstance A and a second ionic substance B, the second ionic substance Bhaving, in the first concentrated solution, a first concentration; asecond injection device for injecting into the preparation conduit, at asecond injection flowrate Q2, a second concentrated solution containingat least the first ionic substance A, the second ionic substance Bhaving, in the second concentrated solution, a second concentrationdifferent than the first concentration in the first concentratedsolution; a control unit programmed to adjust the first injectionflowrate Q1 and the second injection flowrate Q2 in such a way that atany given time the diluted solution resulting from the mixing of theliquid and the concentrated solutions has a desired concentration of thefirst ionic substance A and a desired concentration of the second ionicsubstance B; and a supply conduit for connecting the second end of thepreparation conduit to an inlet of a membrane exchanger, the circulationdevice being arranged on the supply conduit.
 38. System according toclaim 37, further comprising a removal conduit for removing spentliquid, intended to be connected to an outlet of the membrane exchanger.39. System according to claim 37, wherein the first ionic substance Ahas, in the second concentrated solution, the same concentration as inthe first concentrated solution.
 40. System according to claim 37,wherein the concentration of the first ionic substance A in the firstconcentrated solution is less than twice the concentration of the firstionic substance A in the second concentrated solution.
 41. Systemaccording to claim 37, wherein the concentration of the second ionicsubstance B in the second concentrated solution is equal to zero. 42.System according to claim 37, wherein the two concentrated solutions areidentical to each other except that the concentration of the secondionic substance B differs from one solution to the other.
 43. Systemaccording to claim 37, wherein the second ionic substance B ispotassium.
 44. System according to claim 37, wherein the first ionicsubstance A is sodium.
 45. System according to claim 37, wherein thecontrol unit is provided for varying over the course of time the firstinjection flowrate Q1 and the second injection flowrate Q2 of theconcentrated solutions in such a way that the concentration of thesecond ionic substance B in the diluted solution varies over the courseof time in accordance with a predetermined profile.
 46. System accordingto claim 37, wherein the control unit is provided for varying over thecourse of time the first injection flowrate Q1 and the second injectionflowrate Q2 of the concentrated solutions in such a way that theconcentration of the first substance A in the diluted solution variesover the course of time in accordance with a predetermined profile. 47.System according to claim 37, wherein the control unit is provided formaintaining constant the sum of the injection flowrates Q1+Q2 of theconcentrated solutions, in such a way that, for a constant flowrate ofthe liquid in the preparation conduit, the concentration of the firstionic substance A in the diluted solution remains substantiallyconstant.
 48. System according to claim 37, further comprising aninfusion device for infusing a patient with a third solution containingat least one third ionic substance C absent from the treatment liquid.49. System according to claim 48, wherein the third ionic substance C isbicarbonate.
 50. System according to claim 37, wherein the preparationconduit has a first injection point into which the first concentratedsolution is injected, and a second injection point into which the secondconcentrated solution is injected, a first device for measuring aparameter indicative of the concentration of an ionic substance in thetreatment liquid being arranged on the preparation conduit between thefirst injection point and the second injection point.
 51. Systemaccording to claim 50, comprising a second device for measuring aparameter indicative of the concentration of an ionic substance in thetreatment liquid arranged on the preparation conduit downstream of thetwo injection points.
 52. Kit of solutions for extracorporeal treatmentof blood, comprising two concentrated solutions each containing aplurality of ionic substances, the ionic substances and theirconcentrations being identical to each other in the two solutions exceptthat the concentration of an ionic substance B differs from one solutionto the other.
 53. Kit of solutions for extracorporeal treatment ofblood, comprising two concentrated solutions, the first of saidconcentrated solutions containing at least a first ionic substance A anda second ionic substance B, the first ionic substance A being sodium andthe second ionic substance B being potassium, the concentration of thefirst ionic substance A in the first concentrated solution being equalto the concentration of the first ionic substance A in the secondconcentrated solution, the concentration of the second ionic substance Bin the second concentrated solution being equal to zero.
 54. Kit ofsolutions for extracorporeal treatment of blood, comprising twoconcentrated solutions, the first of said concentrated solutionscontaining at least a first ionic substance A and a second ionicsubstance B, the first ionic substance A being sodium and the secondionic substance B being potassium, the concentration of the first ionicsubstance A in the first concentrated solution being less than twice theconcentration of the first ionic substance A in the second concentratedsolution, the concentration of the second ionic substance B in thesecond concentrated solution being equal to zero.
 55. Bag with twocompartments for containing each of the concentrated solutions from thekit according to claim
 52. 56. Bag with two compartments for containingeach of the concentrated solutions from the kit according to claim 53.57. Bag with two compartments for containing each of the concentratedsolutions from the kit according to claim 54.